This invention relates to apparatus for measuring the separation between two light-reflecting surfaces and especially to apparatus using a pulsed-laser technique for measuring the separation between two light-reflecting surfaces.
One pulsed method employed for measuring the separation between optical surfaces uses ultrashort light pulses. If the arrival times of the reflections from the optical surfaces can be measured to sufficient accuracy, then the surface separations can be determined. Picosecond pulses can now be generated but timing circuitry limits the resolution of surface separations to the order of a centimeter at best. In addition, extremely high peak powers are employed in order to provide sufficient energy for detection but these high peak powers can cause air breakdown and possible damage to the optical system being probed.
A second method, closely related to the technique to be described herein, employs high resolution photographic film as the detector. Since high-resolution film is required, this technique is relatively insensitive because the energy necessary to expose these films is quite large. In addition, the available films only operate in the visible region of the spectrum.